The present invention relates to an apparatus and method for optical scanning along a circular path. More particularly, it relates to an apparatus and method of focusing one or more beams of light into one beam spot, scanning this common beam spot across a circular path and receiving light backscattered from this beam spot with one or more detectors. An application of the present invention includes a method for analyzing the number and size of particles suspended in a fluid medium.
The technology disclosed in U.S. Pat. No. 4,871,251 has successfully shown the use of a scanning device to analyze the number and size of particles suspended in a fluid medium. Using the device presented therein, it is possible to characterizes particles in a size range between approximately 1 xcexcm and 1000 xcexcm over a wide range of concentrations by means of measuring the time duration of light backscattered from individual particles inside the fluid medium. However, in the practical use of the patented device it has been found that simultaneous illumination with light of different wavelengths and the analysis of the spectrum of backscattered light could greatly enhance the use of the device.
Instruments defining the current state of the art fall into two categories. One group of optical instruments is concerned with the chemical analysis of ensembles of particles in a fluid medium (Raman spectrometers, Near Infrared spectrometers, and others). These instruments typically do not allow an analysis of individual particles and provide no number or size measurements of individual particles. A second group of instruments analyzes the size and/or number of particles in a fluid medium, either by forward scattering of light (Laser Diffraction Particle Sizers) or measurement of the amount of backscattered light from either an ensemble of particles or individual particles. No simultaneous measurement of chemical composition and size and number of individual particles is available from instruments of this group. The present invention seeks to overcome this limitation.
The present invention shows a novel arrangement for a rotating scanning optics that allows the use of one or more independent illumination and receiving beams, all being focused at the same scanning spot. This scanning spot follows a circular scan path in a plane perpendicular to the axis of rotation of the scanning optics. In addition to the advantages this new arrangement has over the present device based upon that disclosed in U.S. Pat. No. 4,871,251, many other optically scanning devices can benefit from the present arrangement.
The present invention allows the construction of an apparatus for simultaneously analyzing the number, size and chemical composition of individual particles suspended in a fluid medium. The invention is based on the fact that all rays or beams of light entering a focusing lens parallel to its optical axis will be focused at the focal point of the lens on the optical axis.
In the basic form of the apparatus, a first collimated beam of light is focused by the scanning optics into a focal plane. A second collimated receiving beam parallel to the first beam is focused by the same optics into the same focal spot. Both collimated beams are parallel to the optical axis of the scanning optics. The scanning optics can be a single focusing lens or comprise several optical elements that focus incoming light. Using a real focusing lens or lens system, it is not possible to focus incoming light into a single point. Instead the focused beam will form a waist, the smallest waist diameter is determined by the diffraction limit of the focusing optics (Born, Wolf, Principles of Optics). The intersection of the illuminating and receiving beams after passing through the scanning optics defines a measurement volume. The dimensions of this measurement volume are determined by the dimensions of the beam waists of the two focused beams and the intersection angle.
An object that enters the measurement volume is illuminated by the light of the first beam and light will be scattered in all directions. Part of the light falls into the aperture of the second beam and is directed to a detector. If the object enters the illuminating beam outside the measurement volume, no light will be received by the detector.
If the scanning lens is rotated around an axis parallel to its optical axis the measurement volume will scan along a circular path in a scanning plane perpendicular to the axis of rotation. The radius of this circular scanning path is equal to the distance between the optical axis and the rotational axis of the scanning lens. Independent of the angular position of the scanning lens, the two beams always form the measurement volume by intersection; the intersection angle changes slightly along the path.
In a preferred embodiment designed to analyze particles suspended in fluid medium, the optics forming the two collimated beams and the scanning lens are arranged inside an enclosure and the beams fall through an optical window into the fluid medium. The position of the window is chosen so that the focal plane of the scanning lens lays within the fluid medium outside the enclosure.
If the scanning lens rotates with constant angular velocity, the measurement volume will scan through the fluid medium with constant tangential velocity along the circular scanning path. Particles in the fluid medium will enter the scanning path at random locations. The dimension of the scanning volume should be smaller than the size of particles to be analyzed. The measurement volume scans across the surface of a particle suspended in the fluid medium if it enters into the scanning path. For a scanning circle diameter that is much larger than the size of the scanned particle the path across the particle will be almost linear. The length of this path depends on where the particle is hit by the scanning measurement volume and will be between almost zero and the longest linear distance between two points on the surface of the particle. The scanning path across the particle is called a chord. While the measurement volume scans across the particle surface, light backscattered from the particle falls into the receiving optics, guided to a detector and the detector signal is recorded by electronic means. If the relative speed of the scanning measurement volume with respect to the particle is known, the length of the scanned chord is determined by multiplying the relative velocity with the measured time duration of the backscattered light signal. An example of electronics suitable for this task is disclosed in U.S. Pat. No. 4,871,251, which is incorporated herein by reference.
Depending on the application of the apparatus, illuminating beams can have different wavelengths of light; different diameters of beams lead to different illuminating or receiving apertures.